The correct answer is A) incomplete priming. When a centrifugal cargo pump loses suction during operation, it is most likely due to incomplete priming of the pump. Centrifugal pumps rely on creating a low-pressure area at the suction side to draw in the cargo. If the pump is not properly primed, it cannot establish this low-pressure area, causing the pump to lose its ability to suck up the cargo and leading to a loss of suction. The other answer choices are less likely causes. B) the pump running backwards would prevent the pump from working at all. C) the discharge head being too high could cause the pump to stall, but would not lead to a loss of suction. D) head pressure on the suction side is more likely to prevent the pump from starting up in the first place, rather than causing a loss of suction during operation.